1. Field of the Invention
The present invention relates generally to the field of rotary power tools. In particular the present invention relates to a tool-free mechanism for clamping interchangeable rotary tool accessories, such as grinding discs, circular saw blades, etc.
2. Discussion
Various types of rotary power tools require a user to selectively attach a tool accessory, such as a grinding disc. Over the course of a particular project or work period it may be necessary to employ a variety of such accessories and to replace worn or broken ones. Convenient removal and replacement of the interchangeable tool accessories is therefore desirable.
Various methods are employed for clamping such accessory discs. For example, in current grinder designs a replaceable grinding disc has a center hole. The hole in the grinding disc fits over the end of the grinder""s output shaft or drive spindle, with the plane of the grinding disc transverse to the axis of the shaft. For some radial distance the interior surface of the disc hub rests against a flat shoulder of the shaft or an inner washer connected to the shaft.
Conventionally, the grinding disc is clamped in place against the shoulder or inner washer with a retaining-nut threaded down over a threaded end of the drive spindle. An outer washer may be placed between the nut and the hub of the grinding disc. When changing grinding discs, the nut must be loosened and removed. The tightening and loosening of the nut may be partially performed by hand, but the size and shape of the nut make it difficult and/or uncomfortable to properly tighten and then initially loosen using hands alone. Therefore, it is necessary to use a hand wrench or similar tool in order to apply sufficient torque to the retaining nut.
Additionally, when tightening or loosening the nut, the shaft/spindle must be prevented from rotating or else it would be difficult to achieve relative motion between the nut and the rotatable spindle. Conventionally, the loosening and tightening process requires that the operator simultaneously grip both the shaft/spindle and the nut with two separate hand wrenches or the like. Then a torque is applied to the nut while the shaft is held in place. This is an awkward process and made even more so by the necessity to support the weight of the tool itself.
More recently, tools have been designed that incorporate a locking mechanism that locks the shaft/spindle relative to the tool housing. Thus, the rotation of the shaft can be prevented, simply by employing the built in shaft lock. For such a power tool, only one hand wrench is required to loosen or tighten the nut.
The need to use even one hand tool to change grinding discs, however, is still inconvenient. For example, when a replacement disc is required, the tool operator has to stop what he is doing to find a wrench, which may not be readily at hand.
To make the replacement of a grinding disc a tool free process, a number of clamping devices that can be tightened and loosened by hand have been invented. Such prior art devices have had several drawbacks including: the clamping device height interferes with use of the grinding tool; the clamping device does not self tighten during operation or, if it does self tighten, it cannot be readily released by hand; and incorporation of a slip clutch effect to prevent over-tightening.
An example of such a quick acting clamping device is described in U.S. Pat. No. 5,707,275 to Preis et al., which is embodied in the FIXTEC(copyright) Rapid Locking Nut manufactured by INA Wxc3xa4lzlager Schaeffler oHG. The clamping device according to the U.S. Pat. No. 5,707,275, patent has a slip clutch function that, while preventing over tightening of the clamping device, unnecessarily limits the usable torque of the grinding machine.
It is, therefore, one object of the invention to provide an improved device for simple and reliable axial clamping of a tool accessory disc, such as a circular grinding disc, without the use of any hand tools. Further objectives of the present invention include:
A compact, low height design;
Simply shaped and easy to manufacture parts;
Low manufacturing cost;
Provides for self-tightening under load, but without loss of the ability to loosen by hand.
These and other objects of the invention are accomplished according to the present invention in a clamping device constructed of stacked components that can be manipulated from a first position having a first stack height to a second position having a second stack height less than the first stack height. The clamping assembly is installed and tightened in the first position with the greater stack height. After tightening, the axial reaction load on the camping assembly would require significant torque to unscrew. By first manipulating the assembly into the second position and consequently reducing the stack height of the assembly, however, the axial reaction load is significantly reduced. Thus, the torque required to unscrew the device is reduced to levels that can be applied by hand.
In order to so function, the clamping assembly includes a clamp flange positioned axially inward against the hub of the grinding disc. Stacked axially outward of the clamp flange is a first roller cage. The roller cage is pierced by a plurality of angularly spaced perforations. Rotatably mounted in the perforations is a first set of rolling elements. The rolling elements may be needle bearings, which distribute the axial load and minimize the overall stack height of the clamping assembly, but may include other types of bearings. Stacked axially outward of the first roller cage is a center thrust plate, also pierced by a plurality of angularly spaced perforations, equivalent in number to the rolling elements. Equivalently, instead of perforations, appropriately sized and spaced indentations could be located on the opposed sides of the center thrust plate.
Stacked axially outward of the center thrust plate is a second roller cage pierced by a plurality of angularly spaced perforations. Located in the perforations of the second roller cage is a second set of rolling elements. The first and second roller cages are connected so as to rotate together with the two sets of rolling elements in axial alignment. Said connection may be by means of a tab and slot combination on the outer circumference of the roller cages.
The center thrust plate is sandwiched in between the roller cages and can rotate relative thereto. A biasing means, rotationally biases the center thrust plate into a ready or locked position, relative to the roller cages, wherein the two sets of rolling elements rest on the opposite flat surfaces of the center thrust plate, rather than in the perforations of the center thrust plate. The biasing means may include springs.
Stacked axially outward of the second roller cage is a flange nut with an axially projecting hub. The hub threadably engages the shaft of the grinding tool. The clamp flange is rotationally locked with the flange nut, but they have limited axial movement between them. The rotational engagement of the clamp flange and the flange nut may be accomplished by use of mating doublexe2x80x94D surfaces near the radially inner perimeters of the two elements.
Sandwiched rotatably between the clamp flange and the flange nut is a ring collar, which surrounds the stacked center thrust plate and roller cages. The outer circumference of the ring collar has a grippable surface. Within the inner circumference of the ring collar, the interlocked roller cages can rotate. The center thrust plate is connected to the ring collar, so that the thrust plate will turn when the surrounding ring collar is turned. Said connection can be accomplished by means of a tab and slot combination between the inner circumference of the ring collar and the outer circumference of the center thrust plate.
In its ready or locked position the springs have rotationally biased the center thrust plate so that the rolling elements, while inside their respective perforations in the roller cages, are outside the perforations in the center thrust plate and contacting the flat surfaces thereof. Thus, the sub-assembly of the roller cages, rolling elements and center thrust plate will have a first stack height when in said ready or locked position. Accordingly, the entire clamping assembly will have a ready or locked height.
To install the clamping assembly onto a grinder shaft, the user grasps the ring collar and turns it clockwise so as to thread the assembly onto the shaft. The turning of the ring collar is transmitted to the center thrust plate. The rotation of the center thrust plate is transmitted to the flange nut by a torque transmission means. The rotation of the flange nut will cause its threads to move axially down the threaded grinder shaft.
Said torque transmission means allows for some limited rotational movement between the center thrust plate and the flange nut. The torque transmission means may include a bump on the inner perimeter of the center thrust plate that can engage a flat surface on the hub of the flange nut. When tightening in the clockwise direction the bump contacts the flat surface at a first point, but when switched to loosening in the counter-clockwise direction the center thrust plate must rotate relative to the flange nut until the bump engages the flat surface at a second point.
Alternatively the torque transmission means may be in the form of a leaf spring on the inner perimeter of the center thrust plate. When tightening, the inner perimeter of the center thrust plate engages the hub and drives the rotation of the flange nut. When loosening in the counterclockwise direction, however, the leaf spring contacts the hub of the center thrust plate and, by compressing, allows a limited amount of rotation by the center thrust plate relative to the flange nut.
As the clamping assembly threads down on the shaft, the clamp flange will contact the hub of the grinding disc. Further tightening of the assembly will exert a clamping force onto the grinding disc and a corresponding axial reaction force will be transmitted into the clamp flange. The axial reaction force will be transmitted from the clamp flange via the first set of rolling elements, then the center thrust plate, then the second set of roller elements and finally into the flange nut. The flange nut will transmit the axial reaction force into the grinder shaft via its threads.
Although initially only hand tight, operation of the grinder will cause the clamping assembly to self-tighten. During operation, slippage induced rotation of the grinding disc will be transmitted to the clamp flange by friction. The rotation thus imparted to the clamp flange will be transmitted to flange nut and will cause the threads to run down on the grinder shaft, thus increasing the clamping force exerted by the clamping assembly until the slippage of the grinding disc stops.
To loosen the clamping assembly, the user grasps the ring collar and turns counter clockwise. Turning the ring collar integrally turns the center thrust plate. Due to the torque transmission means, as described above, there is some lost rotational motion between the center thrust plate and the flange nut. Initially, therefore, the center thrust plate and connected ring collar can turn without applying the torque that would otherwise be necessary to overcome the axial reaction force on the flange nut.
During that limited range of rotation, the center thrust plate rotates relative to the roller cages. The first set of rolling elements roll between the center thrust plate and the clamp flange and the second set of rolling elements roll between the center thrust plate and the flange nut. Although under compression, the rolling elements roll along the flat surfaces of the center thrust plate until they roll into the perforations therein.
With the rolling elements in the perforations of the center thrust plate, rather than on the flat surfaces thereof, the clamping assembly is in the unlocked position. In the unlocked position the sub-assembly of roller cages, rolling elements and center thrust plate have a second stack height. The stack height of the unlocked position is less than the stack height of the ready or locked position, described above. The thickness of the center thrust plate is such that, with the rolling elements in the perforations of the center thrust plate, rather than on the flat surfaces, the clamping force exerted by the clamp flange is substantially reduced. With the axial reaction force on the clamping assembly correspondingly reduced, the transmission means can now engage the flange nut and readily unthreaded it by hand.
During the loosening operation, the relative rotational movement between center thrust plate and interlocked roller cages causes the compression of the springs. When the flange nut is backed off and the axial reaction force between the elements of the clamping assembly is released, the spring will cause the center thrust plate to rotate relative to the roller cages. The roller elements ride up out of the perforations in the center thrust plate and return to the previously described ready position on the flat surfaces of the center thrust plate. In such ready position, the clamping assembly is ready for reinstallation and tightening
In a particular embodiment, the flat surface of the center thrust plate may include a chamfer on the edge of its perforations. During loosening, the chamfer reduces the rolling friction experienced by the roller elements as they roll across the flat surfaces toward the perforations. This reduction in rolling friction will reduce the initial unlocking torque that the operator must manually apply to the ring collar during loosening, thus making the clamping assembly easier to unlock. The exact shape and dimensions of the chamfer, as well as the number of perforations that have a chamfer, can be selected to achieve the unlocking torque desired by the designer.